1. Field of the Invention
The subject invention relates generally to servo positioning systems, and more particularly to a digital servo positioning system employing a lead/lag integrator to achieve high stiffness during both path tracking and holding.
2. Description of Related Art
Prior art servo position control systems are known wherein a control system moves a mechanical element so as to track a designated path and hold the element at selected points along the path. The concept of "stiffness" in such systems implies a high resistance to external forces which would move the element from its chosen path.
Prior art servo position control systems commonly employ so-called "PID" control wherein an error signal is provided and individual control signals are generated. These individual control signals are typically proportional to the error signal, the integral of the error signal, and the derivative of the error signal. In such systems the integrator which provides the integral of the error signal is typically nested within a feedback compensator, is constrained to integrate the error signal, which typically represents position error.
Either position integration or velocity integration has its drawbacks. Systems which use only position error integration are susceptible to "windup overcompensation," especially during pathtracking, which leads to large position overshoots and oscillations at the end of the move when holding is required. Velocity error integration increases system stiffness during pathtracking, while significantly reducing windup overcompensation compared to position integration. However, velocity integration yields unsatisfactory holding stiffness for many applications. Using both velocity error and position error integration at the same time introduces conflicting control signals under some conditions, which compromises optimal system performance.